Lovely work - small point - there are those here who have made nixie speedos etc. for motorbikes & cars (but not as nice as yours), but most had problems with vibration damaging the nixies and/or their connectors - nixie is parallel to the road and puts pressure on the pins - over time, springs receptacles weaken, dry joints can occur and the nixie itself can become damaged (cathode mounts fail & cathodes/anode shorts).

It might well be worth fitting anti-vibration mounts internally to the unit...

This is a really neat thing. I've been wanting a Nixie display to use
in at least one of my old cars for a long time, but never got up the
ambition to make one. Recently I discovered the fun of connecting a
MegaSquirt EFI controller to an old car to use only for data
logging....of course it could be modified to drive a Nixie display, or
just borrow the input circuitry and programming to make whatever kind
of custom dashboard you want.

A single 4 digit display, with a large rotary switch to select which
parameter to display, might look appropriately old fashioned for a
1950s car.